CROSS-REFERENCE TO U.S. APPLICATIONS
This application is related to U.S. Ser. No. 442,423 filed Nov. 17, 1982 and U.S. Ser. No. 442,424 filed Nov. 17, 1982.
This invention relates to a method and apparatus for sensing the present position of a machine element in a system for controlling the position of the machine element. More particularly, the invention relates to a method and apparatus for sensing the present position for application to a position control system, wherein the present position of the machine element is sensed even after a position control loop is opened by cutting off the machine element from the control system.
In a position control system for positionally controlling a machine element in a machine tool, electric discharge machine or the like, it is necessary to sense the present position of the machine element at all times in order to accurately control the position of the machine element. Such a position control system includes a control unit and a position control loop and is so arranged that the control unit applies move commands having specific magnitudes to the position control loop. Therefore, by monitoring the output of the position control loop, which depends upon the magnitude of the position command, it is possible for the control unit to sense the present position of the machine element. However, in a case where the position control loop is opened by cutting off or isolating the machine element from the position control system and then placing the machine element under the control of another control system, the control unit is no longer capable of sensing the present position. Thus, even when the position control loop is closed again to reconnect the associated position control system, the control unit will not be capable of effecting positional control in an accurate manner.
Accordingly, a method has been proposed wherein the present position of the machine element is sensed on the basis of feedback pulses indicative of the machine element position, even when the position control loop is opened. A problem encountered with this method, however, is that it is difficult to sense the present position accurately because of an offset voltage developed in the associated circuitry.